Dialkyl carbonates are industrially useful as lubricants, fuel additives, and reactive reagents in a wide variety of processes. In particular, dimethyl carbonate (DMC) has found use as an environmentally-acceptable solvent, high-octane oxygenated fuel additive, and esterifying and methylating agent, and the future demand for DMC is projected to exceed current global capacity. (Reviews: B. Schaeffner et al., Chem. Rev. 2010, 110:4554-4581; M. Pacheco and C. Marshall, Energy Fuels 1997, 11:2-29; P. Tundo and M. Selva, Acc. Chem. Res. 2002, 35:706-716.) DMC can also be used as a polymerizing agent in place of phosgene, enabling the production of polycarbonates and polyurethanes by melt transesterification.
Commercial success as bulk industrial solvents or fuels requires methods of synthesis of dialkyl carbonates that are scalable to multi-ton manufacturing, and that avoid costly or hazardous reagents, high pressures and temperatures, chemical wastes, and low-value by-products. In particular, methods that do not employ phosgene, chloroformates, or similarly corrosive and/or toxic intermediates are of considerable industrial value.
The industrial synthesis of dimethyl carbonate by reaction of methanol with phosgene has been largely displaced by a less hazardous and more environmentally benign process based on the catalytic oxidative carbonylation of methanol (Scheme 1):

The carbonylation process is usually carried out using a copper(II) catalyst. The method was first reported by T. Saegusa et al., J. Org. Chem. 1970, 35, 2976-2978, refined by E. Perrotti and G. Cipriani (U.S. Pat. No. 3,846,468 (1974) and U.S. Pat. No. 3,980,690 (1976), and commercialized by Enichem in 1983. The so-called “Enichem Process” has been subject to an ongoing series of improvements and refinements ever since. (U. Romano et al., Ind. Eng. Chem. Prod. Res. Dev. 1980, 19:396-403; Z. Kricsfalussy et al., Ind. Eng. Chem. Res. 1998, 37:865-866.) Diethyl carbonate has been prepared by this means as well (B. Dunn et al., Energy Fuels 2002, 16:177-181; H. Xiong et al., Ind. Eng. Chem. Res. 2009, 48:10845-10849.) Cyclic carbonates have likewise been prepared from carbon monoxide and diols, using a palladium(II) catalyst (P. Giannoccaro et al., Organometallics 2006, 25:2872-2879.) W. Gaenzler et al., in U.S. Pat. No. 4,113,762 (1978), disclosed catalysts containing complexes of CuCl with chlorides of V, Cr, Fe, Co, Al, and Si. J. Hallgren, in U.S. Pat. No. 4,361,519 (1982), disclosed the use of redox catalysts based on Mn or Co, in combination with Ru, Rh, Pd, Os, Ir or Pt metals or complexes. The use of a gold/carbon anode in an electrochemical process for methoxylation of carbon monoxide has been reported (A. Funakawa et al., J. Phys. Chem. B 2005, 109:9140-9147.)
The direct synthesis of DMC from CO2 and methanol is an attractive route, in theory, due to the low cost of CO2 and the environmental desirability of processes that consume it, but the kinetic and thermodynamic stability of CO2 are obstacles to efficient conversion that have yet to be overcome.
The copper-catalyzed reactions involve dissolved copper species such as Cu(OMe)Cl which tend to be poorly soluble in organic solvents. As a result, slow conversion rates and deactivation and leaching of copper(II) catalysts by co-product water are persistent problems, particularly in slurry processes, where the removal of water is difficult. Continuous-process variants have been developed, involving the energy-intensive volatilization, condensation, and isolation of product, and recycling of reactants (N. Di Muzio et al., U.S. Pat. No. 5,210,269 (1993)).
As is the case with most industrial-scale syntheses, a continuous oxidative carbonylation process employing heterogeneous catalysts would be much preferred, and continuous gas-phase processes using Co catalysts have been reported (D. Dreoni, D. Delledone et al., U.S. Pat. No. 5,322,958 (1994) and U.S. Pat. No. 5,457,213 (1995)). The existing gas phase processes for the oxidative carbonylation of alkanols are handicapped by low conversions, high pressures and/or high temperatures, and undesired by-products, as well as the hazards associated with potentially explosive oxygenated feed mixtures. There remains a need for efficient, cost-effective, heterogeneous catalytic syntheses of dialkyl carbonates.
For these and other reasons, there is ongoing research into alternative processes, catalysts and catalyst supports for DMC production. Several investigators, including the present inventors, have previously demonstrated that oxygen adsorbed onto gold and silver surfaces is activated toward the oxidation of a variety of substrates. (B. Xu, L. Zhou, R. J. Madix, C. M. Friend, Angew. Chem. Int. Ed. Engl. 2010, 49:394-398; X. Liu, R. J. Madix, C. M. Friend, Chem. Soc. Rev. 2008, 37:2243; I. E. Wachs, R. J. Madix, Surf. Sci. 1978, 76:531; D. M. Thornburg, R. J. Madix, Surf. Sci. 1990, 226:61; J. L. Gong, T. Yan, C. B. Mullins, Chem. Commun. 2009, 761). The ability of gold to catalyze various oxidative reactions of carbon monoxide has been noted previously (M. A. Bollinger and M. A. Vannice, Appl. Catal. B: Env. 1996 8:417-443; W. Deng et al., Appl. Catal. A: Gen. 2005 291:126-135; Q. Fu et al., Chem. Eng. J. 2003, 93:41-53; Y. Tai et al., Appl. Catal. A: Gen. 2004, 268:183-187; F. Bocuzzi et al., J. Phys. Chem. 1996, 100:3625-3631; F. Fajardie et al., PCT Intl. Appl. WO 2005/089937), and the carbonylation of methanol to form methyl formate has been carried out with gold catalysis (A. Wittstock et al., Science 2010 327:319.) It has not been previously known, however, that gold can serve as a catalyst for the oxidative carbonylation of alkanols with carbon monoxide and oxygen, to form dialkyl carbonates with high selectivity.